RE: [RFC] add sysfs to dynamically control blk request tag maintenance

[Chen, Kenneth W]

Jens Axboe wrote on Friday, October 07, 2005 12:24 AM
> I don't understand the need for this patch - the generic tagging is
only
> used if the SCSI LLD indicated it wanted it by issuing a
> scsi_activate_tcq(). So blk_queue_start_tag() is only called if the
LLD
> already did a scsi_activate_tcq(), and blk_queue_end_tag() is only
> called if the rq is block layer tagged. blk_queue_find_tag() is only
> used with direct use of scsi_find_tag(), a function that should (and
is)
> only usable by users of the generic tagging already.
> 

You beat me by a couple of minutes.  I was about to say that the culprit
is in the qla2x00 driver where it unnecessarily activated generic blk
tag queuing by calling scsi_activate_tcq() and it never uses tag.

> So please, a description of what problem you are trying to solve would
> be appreciated :-)

It starts out with scsi_end_request being a fairly hot function in the
execution profile, then I noticed blk_queue_start/end_tag() are being
called but no actual consumer of using the tag.  I'm trying to find a
way to avoid making these blk_queue_start/end_tag calls.  I got the
answer
now. The proper way is to fix it in the scsi LLDD.  Scratch this patch,
new patch to follow :-)

- Ken
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